Arc lamp and control mechanism therefor



Oct. l, 1940. P. MoLE r-:r AL

ARC LAMP AND CONTROL MCHANSM THEREFOR Filed NOV. 18, 1958 3 Sheets-Sheet l .N @bwl A'ITGRNEY.

oct. l. 1940. P. MOLE ETAL ARC LAMP AND CONTROL MECHANISM THEREFOR Filed Nov. 18, 1938 3 Sheets-Sheet 2 NNN Oct. 1, 1940. P. MoLE Er AL 2,216,352

ARC LAMP AND CONTROL MECHANISM THEREFOR Filed Nov. 18, 193e s sheets-sheer s Patented Oct. 1, 1940 UNITED STATES PATENT OFFICE ARC LAMP AND CONTROL MECHANISM THEREFOR Application November 18, 1938, Serial No. 241,220

Our invention relates to arc lamps and has particular reference to an arc lamp embodying control features adapting ity particularly for such applications as the projection of colored motion picture films, and the like. n

The recent major developments and improvements in connection with colored cinematography have given rise to numerous problems in connection with the projection of colored motion lo pictures and the lighting of motion picture sets.

One of the most perplexing problems regarding the use of arc lamps confronting the motion picture industry today is in connection with a process known in the trade as back projection.

w A great deal of difliculty has been encountered in the use of this process even in connection with the conventional black and white photography ybecause of the lack of projection lamps having the ability of providing sufficient illumination of the required uniformity as regards quality and -0 quantity of light. Whenever the picture is to be photographed in color, the problem becomes even more acute inasmuch as the fidelity of color rendition in the present commercial color photographic processes depends to a very large extent upon the uniformity of quality of light used for the projection of the colored backgroundk upon the translucent back projection screen.

As a result of the highly sensitive nature of the color films as respects quantity and quality of illumination, the quality of the light emitted by the crater of an arc lamp, and the location of this crater relative to the principal light projecting optical` system included in the lamp, must be carefully controlled. y The control equipment for maintaining the proper relations as regards crater location, length of arc and current consumption by the lamp are at present not sufficiently accurate in their operation/to insure constancy o1' results in that the power used by thearc varies beyond permissible limi-ts and the mechanism for maintaining the location of the crater at a fixed point is not sutilclently sensitive in its operationand is too slow in response to changes in crater location.

It is therefore a primary object of our invention to provide such control mechanism for arc lamps as will insure the accurate maintenance of a uniform, high standard of quality and quantity of light projected by the lamp.

An additional object of our invention is to provide a control mechanism for an arc lamp for accurately and automatically maintaining the crater of the positive electrode at a fixed location relative to the principal light projecting system employed in the lamp.

It is also an object of our invention to provide an arc lamp of the character set forth, wherein an extremely sensitive and rapidly responsive photo-electric means is employed to maintain the crater of the positive electrode at a fixed location relative to the light projecting means.

Another additional object of our invention is to provide an arc lamp of the character set forth in the preceding paragraphs in which the sensitivity of the photo-electric control mechanism and the desired location of the crater relative to the light projecting means may be readily adjusted.

It is a further object of our invention to provide in an arc lamp which includes a photo-electric control mechanism of the character set forth 15 a control mechanism for maintaining the power used by the arc at a specified fixed value and thereby maintain a uniform quality and quantity of light to be emitted by the lamp.

It is an additional object of our invention to 20 provide an arc lamp of Ithe character set forth in the preceding paragraphs with a novel brush mechanism for conveying the arc current to a rotating positive electrode.

Other objects and advantages of our invention 25 will be apparent from a study of the following specifications, read in connection with the accompanying drawings, wherein:

Fig. 1 is a side elevation of one form of a projection type arc lamp constructed in accordance with our invention and with parts broken away to show parts of the mechanism in vertical, longitudinal section;

Fig. 2 is a fragmentary plan view, partly in section, of a projection type arc lamp showing $5 the relation of the positive electrode holder, the light projecting means of the lamp, and the control mechanism for maintaining the positive crater at the proper location relative to the projecting means;

Fig. 3 is a front elevational viewI partly in section, showing the upper portion of the positive electrode supporting and feeding mechanism when viewedffrom the approximate location of the optical system of the lamp; 45V

cludes a base member I3 upon which is supported a gear housing II by means of a box-like supporting member I2 suitably attached to the housing II and to the base member I and insulated from the gear housing II by means of suitable insulating material I3 interposed between these members.

The gear housing II is adapted to rotatably and removably support a positive electrode I4 and is accordingly fitted with an electrode-receiving sleeve I5 which is journaled for axial rotation as by means of ball bearings I3 and I1 carried by suitable supporting means within the housing II.

Rotary motion is imparted to the sleeve I5 and to the electrode I4 carried thereby by means of a motor I8 supported from the underside ol.' the base I0 and coupled to the sleeve I5 by means of reduction gearing I3 interconnecting the motor I3 and upwardly extending interconnecting shafts and 23a which are in turn coupled to the sleeve I5 by means of bevel gears 2| and 22. The gear ratios of the reduction gearing I9 and the bevel gears 2I and 22 are selected with respect to the speed of rotation of the motor I3 so as to deliver an optimum speed of rotation to the positive electrode I4.

In order that the sleeve I5 may be held against axial movement during the rotation thereof, we provide a thrust ring 23 which is suitably secured to the sleeve I5 in such a position as to engage one face of the inner race of the ball bearing I6, the other face of which engages a hub of the bevel gear 22.

The vertically extending shafts 20 and 20a which interconnect the motor I3 and the sleeve i5 may be journaled for rotation by -suitable bearings 24, 25 and 2G provided in the base plate Il) and the supporting structure I2 and may be held against axial movement by means of suitable thrust collars and/or washers applied to the shafts 23 and 20a in positions engaging the previously described bearings. The shafts 20 and 217g may be interconnected by means of an insulating coupling 26a.

The motor i3 is preferably connected, as shown in Fig. 4, between line conductors LI and L2 which are connected to a suitable source of power, together with a governing mechanism for causing the motor I3 to rotate at an essentially constant speed. This connection may include a circuit 21 which extends between the line conductors LI and L2 and includes a field IBF of the motor I3. An armature IBA of the motor I3 is preferably connected by means of a conductor 23 to the line conductor L2, the other side of the armature IBA being connected by means of a conductor 23 to a speed limiting resistor 30 which is in turn connected to the line conductor LI by means of a conductor 3|. The armature current which flows through the circuits 23, 23 and 3l causes a voltage drop in the resistor 30 so as to apply a voltage across the armature IBA which is less than the voltage supplied by the conductors LI and L2.

We include a governor mechanism 32 for the motor I3 which may be of the well known flyball type connected for rotation by the shaft of the motor I3 and which includes contacts 32e which are arranged to be opened by the mechanism 32 whenever the speed of rotation of the motor I3 reaches a predetermined value. In order that the opening oi' these contacts may be used to control the speed of rotation of the motor I3, we connect the contacts 32e in a circuit 33 which extends between the line conductor LI and the previously described conductor 23 so that the contacts 32e a-e connected in parallel with the speed limiting resistor 33. It follows therefore that if contacts 32e are closed, full line voltage will be applied to the armature I3A and as the motor I3 accelerates the contacts 32c will open at a predetermined speed so as to insert the series resistor 30 in the armature circuit. The consequent reduction in the armature voltage will allow the motor I3 to tend to slow down, such slowing down being arrested by the reclosing of the contacts 32o. It follows therefore that the contacts 32C will rapidly open and close so as to maintain the speed of rotation of the motor I3 at a value corresponding to the speed at which the contacts 32e open.

We include a capacitor 34 in a shunt circuit 35 paralleling the contacts 32e so that the tendency for the current flowing through circuit 33 to persist during the opening operation of contacts 32c and cause arcing thereof and burning of the contact points will be absorbed by the capacitor 34 and provide long life, trouble-free operation of the governor mechanism 32.

During the operation of the lamp, the electrode I4 will be progressively consumed so that the electrode I4 should be fed forwardly toward the other electrode of the lamp if a crater 33 (see Fig. l) which is formed'in the end thereof. is to be maintained in a fixed location relative to the optical system of the lamp. We therefore provide an electric motor 31 which is mechanically connected to the electrode I4 in such a manner as to cause the electrode I4 to be fed toward the other electrode in terms of its rate of consumption.

This mechanical connection may include interconnected vertical shafts 33 and 33a which are coupled to the electric motor 31 by means of suitable reduction gearing 33 and which is also coupled to a scroll gear 40 by means of bevel gears 4I and 42, the first of which is secured to the shaft 33a and the latter of which is secured to the scroll gear 40 by means of suitable interconnecting mechanism, such as pins 43 engaging the hubs thereof. The scroll gear 40 and the bevel gear 42 connected thereto are rotatably supported on the sleeve I5 and are disposed on opposite sides of the ball-bearing I1 so that axial movement of these gears along the sleeve I5 is prevented. The shafts 33 and 33a may be journaled for rotation by means of bearings 44, and 43 carried by the base plate III and the supporting structure I2 and may be prevented from moving axially by means of suitable thrust collars and/or washers applied to the shafts 33 and 33a in relative positions to engage the aforementioned bearings. The vertical shafts 33 and 33a may be interconnected and insulated from each other by means of an insulating coupling 43a.

The rotation of the scroll gear 43, imparted thereto by the motor 31, may be converted to produce an advancing translatory movement of the electrode I4 by means of feeding wheels 41 and 43 which are supported upon a pair of axles 43 and 53 carried in slotted bearing brackets 3| attached to a face plate portion 32 of the sleeve I5. The feeding wheels 41 and 43 are preferably formed with gear teeth on their periphery arranged to engage the scroll teeth on the scroll gear 40 so that as the scroll gear 40 is rotated relative to the sleeve I5, the feeding wheels 41 and 43 will be rotated thereby and feed the elec- 75 trode I4 in a forward, axial direction toward the other lamp electrode. Suitable tension springs (not shown) interconnect the axles 43 and 56 for normally urging the feeding wheels 41 and 46 into gripping engagement with the electrode I4.

We prefer to make the speed of rotation of the motor 31 and thegear ratio of the mechanical connection to the electrode feeding wheels 41 and 46 such that the electrode I4 will normally be fed forwardly toward the other lamp electrode at a rate slightly lower than its rate of consumption during the operation of the arc. In order that this speed of rotation of the motor 31 may be maintained, we provide a governing mechanism 53 which may be of conventional construction and supported by the frame of the motor 31 and coupled to the shaft thereof.

We provide a controlling mechanism for superseding the control applied by the governing mechanism 53 so as-to accelerate the speed of the motor 31 in terms of the tendency of the crater 36 to recede from its proper or predetermined desired location relative to a principal light projecting means of the lamp which may include a pair of condensing or projection lenses 64 and 65 (see Fig. 2), supported in a suitable lens mount 56 from the lamp housing 61 which encloses the previously described mechanism. The control mechanism includes a'photoelectric or photronic device 56 supported by a supporting plate 59 secured to the lamp housing 51 in such a position as to intersect a control beam of light transmitted from the crater 36 lthrough a positive lens 60 and reflected by a mirror 6| so that changes in the intensity of illumination of the photo-electric device 66, as a result of movement of the crater 36 relative to the light projecting system, may vary the electrical characteristics thereof and effect a change in speed of the motor 31.

The lens 60 is preferably located with respect 4to the predetermined desired location of the n lamp.

The mirror 6I is interposed between the lens 60 and the photo-electric device 66 so that the beam of light from the crater 36 may be turned into approximate parallelism with the side of the lamp housing 51 and provide a convenient compact arrangement of the photo-electric device 56 and a filter 62 and masking plate 63 associated therewith. The parts are preferably arranged relative to each other so that the light from the crater 36 first passes through the lens 66, is reflected by the mirror 6i and caused to pass through the filter 62, which removes the infra-red or heat rays, and fall upon the masking plate 63 which is so located so that the lens 60 forms a real image of the crater 36 upon the masking plate 63.

The masking plate 63 is provided with an adjustable orifice or slit 64, adjustment of the area of which is obtained by means of a control knob 465 (Fig. so that manual adjustment may be made of the amount of light which can be passed by the slit 64, the light then passing through the slit 64 and through a diffusing screen 66 to impinge upon the photo-eletcric device 56. The dif- 1| fusing screen 66 operates to so diffuse the light passing through the slit 64 and impingin upon the photo-electric device 66 as `to reduce its concentration thereon to a point where danger of undesirable local heating or paralysis of the device 56 is obviated.

It will be readily apparent to those skilled in the art that the optical system just described constitutes an optical lever and valve arrangement which operates as if the beam of light travelling from the crater 36 were a lever pivoted at the optical center of the lens 66, and should the crater 36 be moved rearwardly from the position illustrated in Fig. 2, the beam of light will be caused to pivot about this pivot point and cause the real image of the crater 36 to move laterally relative to the slit 64. This lateral movement of the image of the crater 36 causes a part of said image to enter the slit 64 and transmit an increased quantity of light therethrough to impinge upon the photo-electric device 56.

In Fig. 4 we have illustrated diagrammatically one system of electric connections which may be employed with the photo-electric device 66 to provide the type of control previously described. A fleld winding 31F of the motor 31 is connected for constant excitation to the supply conductors LI and L2 and an armature 31A of said motor is also connected between the supply conductors LI and L2 in series relationship with a speed limiting resistor 61.,

In order that the speed of the motor 31 may be maintained at a substantially constant, normal value, we connect contacts 53e of the governing mechanism 53 in a circuit paralleling the speed controlling resistor 61 and arranged these contacts in such a manner that whenever the speed of rotation of the motor 31 drops below a specified minimum value these contacts will close and short-out the resistor 61 so as to apply full line voltage to the armature 31A ofthe motor 31 and cause an acceleration in the speed of e rotation thereof.

The governing mechanism 53 is preferably arranged so that the speed of rotation of the motor 31 at which the contacts 53o close is of very nearly the same value as the speed at which these contacts open so that the motor 31 is caused to operate at a substantially constant speed due to very rapid opening and closing of the contacts 53C, tending to very rapidly accelerate and deaccelerate the speed of vrotation of the motor 31 and thereby maintainvan essentially constant average speed of rotation.

We also provide a capacitor 66 connected in shunt relationship with the contacts 530 so that inductive discharges occasioned by the opening of these contacts maybe absorbed to prevent undue arcing and burning of these contacts.

The speed of the motor 31 is accelerated whenever the crater 36 recedes from its predetermined location a distance suilicient to increase the intensity of illumination on the photo-electric device 56 andenergize a sensitive relay SR associated therewith which in turn energizes a control relay CR so as to short-circuit the speed regulating resistor 61. These connections include a pair of conductors 69 and 10 which connect the photo-electric device 56 to the coil of the sensitive relay SR. The photo-electric device 56 is preferably of a well known typewhich generates an electric potential, the magnitude of which is proportional to the amount of light impinging on the photo-electric device 56.

Normally open contacts SRa of the sensitive relay SR are included in a circuit 1I which extends between the conductors LI and L2 and inycludes the coil of the control relay CR. Normally open contacts CRa of the control relay CR are included in a circuit 12 which parallels the speed controlling resistor 61 so as to provide a direct connection between the conductor LI and the armature of the motor 31 whenever this relay is energized. A discharge resistor 13 is connected by means of a conductor 14 in parallel with the contacts SRa to prevent undue arcing and burning of these contacts.

summarizing briefly, the motor 31 normally operates at a constant speed controlled by the gov, ernor mechanism 53 at a value lower than that required to feed the electrode I4 forwardly at the same rate as it is consumed by the arc. The crater 36 thereof gradually recedes from the light projecting means and causes an increase in the amount of light impinging on the photoelectric device 58. This increase in the quantity of light causes an increase in the potential generated by the photo-electric device 58 and energizes the sensitive relay SR and in turn the control relay CR so as to short-out the speed limiting resistor 61 and cause the motor 31 to rotate at a speed higher than its governed speed. The high speed operation of the motor 31 will thereupon cause the electrode I4 to be advanced at a rate higher than its rate of consumption and thus advance the crater 36 toward the light projecting means. This re-advance of the crater 36 Will decrease the amount of light falling upon the photo-electric device 58 and cause the reverse operation of the relays associated therewith so that the speed of the motor 31 is again reduced to the value main,- tained under normal conditions by the governing mechanism 53. The electrode I4 is therefore fed continuously forward by the motor 31, the speed of feeding being accelerated from time to time to maintain the crater 36 in a substantially fixed location relative to the light projecting means.

It is desirable that this fixed location of the crater 36 relative to the light projecting means may be suitably adjusted, and we accordingly provide means therefor which is manually operable to shift the slit 64 transversely of the control beam of light. We also incorporate with this means a mechanism for adjusting the area of the slit 64 to regulate the maximum amount of light which may be transmitted therethrough.

These control devices are illustrated in Fig. 5 and include a bracket member 15 secured to the lamp housing 51 and adapted to slidably support the masking plate 63 for translatory movement transversely of the control beam of light. Translatory motion may be imparted to the masking plate 63 and the slit 64 therein by means of a control knob 15a coupled to a feed screw 15b which coacts with the lamp housing 51 and a nut 15e secured to the masking plate 63 to move said masking plate 63 transversely in said guide bracket 15 Whenever the control knob 15a is rotated. A shift in the position of the slit 64 caused in this manner will alter the location which must be assumed by the crater 36 in order to cause the control beam of light therefrom to pass through the slit 64 and impinge on the photo-electric device 56 to actuate the controls in the manner previously described.

In order that operation of the previously described control knob 65 will adjust the area of the slit 64 to regulate the sensitivity of response of the photo-electric device 56, we secure the knob 65 to a feed screw 16 which is rotatably supported by a bracket 16a attached to the masking plate 63. The feed screw 16 coacts with the bracket 16a and a nut 16D secured to a gate 16e which is slidably guided for movement over the slit 64 by means of guides 16d so as to move the gate 16c to obscure more or less of the slit 64 as the knob 65 is rotated. It is readily understood that adjustment of the area of the slit 64 also adjusts the sensitivity of response of the photo-electric device 56 since a change in the area of said slit requires that the image of the crater 36 move a greater or less distance Idepending upon whether the slit area is increased or decreased) into the slit 64 to transmit the same amount of light to the photo-electric device 56 as was transmitted before the slit area was changed.

It will be readily understood by those skilled in the art that should it for any reason be found unnecessary to rotate the electrode I4 in order to maintain a symmetrical form of the crater 36, the means for rotating said electrode may be disconnected or omitted and the speed of the feeding motor 31 and the gear ratio of the mechanical connection between the motor 31 and the feeding wheels 41-46 may be accordingly changed to provide the proper rate of feed for the electrode I4 as dened hereinbefore.

Referring again to Fig. 1, we have illustrated therein an externally operable mechanism for effecting the release of the electrode I4 from the elctrode feeding wheels 41 and 46. This mechanism may be constructed in accordance with the details disclosed in a co-pending application, Serial No. 227,142, filed August 27, 1938, by Fielding C. Coates, which has matured into United States Letters Patent 2,174,736 on October 3, 1939, and includes a slidable sleeve 11 which is disposed between the sleeve I5 and the electrode I4 and carries at its forward end a wedging member 16 adapted to engage rollers 16 and 60 mounted upon the ends of the axles 46 and 56 so as to wedge these axles apart and free the feeding wheels 41 and 46 from the electrode I4 whenever the wedging member 16 is moved rearwardly. This rearward movement of the wedging member 16 may be accomplished by the inter-action of a cam and lever arrangement 6I-62 with a collar 63 secured to the rearward end of the slidable sleeve 11.

To facilitate the assembly of the mechanism previously described, we provide a hinged cover 64 for the gear housing I I which may be hingedly supported on the housing II by means of a hinge pin 65 and an opening 66 in the rearward end of the gear housing Il. In order to reduce the tendency for carbon dust, dirt, and other foreign material to enter into this opening and in order to facilitate the insertion of new positive electrodes I4, we provide a funnel-shaped bushing 61 which is adapted to be secured in and partially fill the opening 66.

We also provide a mechanism or heat shield for conducting the heat generated in the arc away from the control and feeding apparatus and which includes also means for cooling the body portion of the electrode I4. The heat shield includes a water jacket 66 which may be formed of cast material and secured to the housing Il by means of bolts or studs 66a, 66h and 66c. The water jacket 66 is provided with water circulating passages 66, 90, 9i and 92. Cooling water is supplied through a suitable water supply connection 63 communicating with the water passage 66- and is discharged through a water outlet (not shown) communicating with the water passage 92. As best shown in Fig. 3, the cooling water preferably enters by means of the water supply connection 93, travels laterally through the interconnected circulating passages 89, 90, 9| and 92 in sequence and absorbs the heat from the water jacket 88 and the mechanism connected thereto.

A means is provided for conducting the power current from the external source to the electrode I4 which includes a pair of electrode brushes 94 and 95 enclosed by the water jacket 88 and disposed on either side of the electrode I4 and slidably mounted in guides 99 formed in the water jacket 99 and retained in these guides by means of a cover plate 91 secured to the water jacket 88 as by means of screws 98.

We provide a manually operable adjusting means for adjusting the location of the brush 94 relative to the electrodeA I4 so that the position of the brush 94 may be varied at will to accommodate varying sizes of electrodes I4.- This adjusting mechanism includes an adjusting nut 99 which is threadedly engaged with a boss |00 secured to the water jacket 88 and is provided with |09 on the brush 94 and is coupled to the adjustf ing nut 99 by means of an adjusting screw |04 threadedly engaged with the nut 99 and locked in position by means of a lock nut |09. A compression spring |09 is confined in la recess I0I` provided in the exterior end of the boss |00 and forces the adjusting nut 99 away from the ,ad-

justing rod |02 so as to maintain a frictional engagement in the threaded connection of the nut 99 and the boss |00 to prevent accidental rotation thereof. It will be seen that rotation of the ad- Justing nut 99 will cause it and the adjusting rod |02 to move inwardly toward the electrodes I4 or outwardly away therefrom so as to place the brush 94 in different positions corresponding to varying diameters of the electrode I4. f

We prefer to form the electrode contacting face of the brush 94 as a plane surface disposed parallel to the axis of the electrode I4 and the electrode contacting end of the brush 99 as a V shaped notch |09, so that the inter-action oi' the brushes 94 and 99 bearing against the electrode I4 will operate to hold the electrode I4 in a cen-- tered relationship relative to its axis of rotation.

We provide a means'for yieldably forcing thc brush 99 into contact with the electrode I4 which includes a lever |09 pivotally mounted to the gear housing II and carrying a forked end |I0 which encompasses a stud I I attached to the outer end of the brush 99. A pair of inwardly extending pins ||2 and ||9 are carried by the forked end ||0 and engage an annular groove ||4 which is provided on the exterior portion of the stud III The lever |09 is urged in a counter-clockwise direction, as viewed in Fig. 2, by means of a compression spring I |9 which is confined between the side of the gearv case and the other end of the lever |09 by means of washers II6 carried by a pin ||1, suitably insulated from lever |09. so as to force the brush 99 into yielding engagement with the electrode I4.

It is apparent, of course, that if a spent electrode I4 is removed, the brush 95 may move inwardly a distance suiiicient to block the opening in the water jacket 88 provided'for the electrode I4 so as to prevent the insertion of a new electrode. We therefore provide a handle portion I9 on the lever |09 to allow the operator to manually rotate the lever |09 to move the brush 95 outwardly to permit the insertion of a fresh electrode.

In order that the power for the arc lamp may be suitably connected to the brushes 94 and 95,

-we provide for the brush 94 a flexible electrical conductor I I9 which is preferably maintained in intimate electrical contact with the brush 94 by means of the previously described bearing surface or bolt head |09. The conductor II9 extends to the exterior of the water jacket 99 where it may be suitably secured to the body portion of the water jacket 89 by means of a terminal bolt |20 so as to provide attachment to an electrical supply conductor.

In a similar manner we provide for the brush 95 a flexible electrical conductor I 2| which is electrically connected to the brush 99 by means of the stud and which extends exteriorly of the water jacket .89 and is suitably secured to the body portion of the water jacket 99 as by means of a terminal bolt |22. The conductors I|9 and |2| may be inter-connected electrically by means of an external jumper (not shown) connected between the terminal bolts |20 and |22 so that a power supply conductor may be connected to either of these terminal bolts. n

In order that the length of travel of the brushes 94 and99 in the guides 99 may be limited to a reasonable value so as to avoid the imposition of unnecessary strains upon the flexible conductors II9 and |2I, we provide each of the brushes 94 and 99 with a dowel-pin |24, the exposed ends of which slide is short slot-like recesses |29 provided in the body portion of the water jacket 99 so that the travel of the brushes 94 and 99 will be limited to the length of the slots or recesses We provide a negative electrode |28 for the arc lamp and support it on a negative electrode supporting structure |21 in such a manner that its axis may be adjusted in position relative to the axis of the positive electrode I4 through the employment of a carriage '|28 slidably carrying the supporting structure |21 and slidably supported by the base I0. An extension portion |29 of the base plate I0 slidably supports the carriage |29 on ways |90 which may be similar in shape and construction to the ways employed on the beds of lathes, or other machine tools for slidably supporting the carriages thereon. In order that the carriage |29 may be adjusted in its position along the ways |90. we provide a threaded shaft |9| which is secured to the carriage |28 by means of a set screw |92 and which carries an internally threaded bevel gear |99 threaded upon the rod |9I. The bevel gear |99 is provided with a hub portion |94 arranged to contact a downwardly depending flange portion |99 of the base extension |29 and a sleeve, or spacer member |99 which encircles the rod |9| and bears against the face ment of the sleeve |36 with the face of the gear |33 and the flange |31.

Rotation of the bevel gear |33 will therefore cause the shaft |3| to move relative to the base extension |29 and slide the carriage |28 in the ways |30 so as to provide a longitudinal adjusting movement to the negative electrode |26 supported thereby. An externally operable remote control means not shown) is preferably provided which includes a bevel gear (not shown) engaging the bevel gear |33 so that the previously described adjustment may be accomplished by performing the required operation exteriorly of the lamp housing 51.

In order that a similar adjustment of the position ofthe negrive electrode |26 may be made in a direction transverse to the axis of the positive elect-rode |4, we slidably secure the negative electrode supporting structure |21 to the carriage |28 by means of ways |38 so that it may be moved therealong by rotation of a feed screw |39 carried by the supporting structure |21 and engaging a nut |4|] carried by the carriage |28. The feed screw |39 is also preferably extended exteriorly of the lamp housing 51 in any suitable manner so that such adjustments afforded by this means may be readily made.

The supporting structure |21 includes a negative electrode holder |4| which is slidably mounted upon guide rods |42 extending between upper ends of the arm portions of an angularly disposed U shaped portion |43. A portion |44 of the negative electrode holder |4| is secured to a lower portion |45 thereof in any suitable manner and electrically insulated therefrom by insulating material |46. The portion |44 carries an electrode clamping device which may include a` suitable electrode receiving recess |41 and an electrode clamping screw |48 cooperating therewith to secure the electrode |26 in the recess |41. An electrical terminal |48 is provided for the attachment of an incoming power line conductor.

In order that the negative electrode feeding mechanism may be protected from the intense heat generated in the arc, we provide an insulating guard |50 which is secured to the arm of the U shaped portion |43 which is disposed nearest the arcing end of the negative electrode |26, The insulating guard |56 may be constructed of a. refactory or heat insulating material, such as asbestos or its compounds.

A means for moving the negative electrode holder |4| upwardly along the guide rods |42 is provided for maintaining constant length of arc established between the negative electrode |26 and the positive electrode |4 which includes a reversible electric motor |5| (not Shown except in the wiring diagram, Fig. 4) mounted in any suitable location on the base plate I0 or the extension |29 thereof and mechanically connected to the negative electrode holder |4| through a shaft |52 rotatably supported by the electrode supporting structure |21. The shaft |52 is disposed at the lower end of the electrode supporting structure |21 and bears a sprocket |53 secured thereto so as to be rotated thereby. An endless chain |54 extends about the sprocket |53 and about another sprocket |55 rotatably supported by a shaft |56 secured to the upper end of the electrode supporting structure |21 so that rotation of the shaft |52 will also cause a rotation of the endless chain |54.

We provide a depending tooth |51 on the lower portion |45 of the negative electrode holder |4|, the tooth |51 being so disposed as to be engaged by the upper half of the endless chain |54 so that movement of the upper portion of the endless chain |54 will cause the electrode holder |4| to be moved along the guide rods |42.

The manner in which the motor |5| cooperates with other previously described equipment to maintain the desired arc conditions may best be understood by reference to the wiring diagram. Fig. 4. The arc lamp is connected to a suitable source of power by means of the line conductors Ll and L2, The line L| is preferably connected directly to the positive electrode Il, while the line L2 is connected to the negative electrode |26 through a suitable ballast resistor |56 by means of a conductor |59.

The reversible electric motor |5| is connected in a bridge circuit in such fashion that the motor |5| will operate to maintain the resistance of the arc at any desired value. A field |5|F of theA motor |5| is connected in a circuit |60 which extends between the line conductors LI and L2 and includes an adjustable field rheostat |6|, and an armature |5|A of the motor |5| is connected as the balance responsive device in a. bridge circuit comprising the are circuit LI-ISS-LZ, and a. reference circuit |62 which extends between the conductors L| and L2 and includes a tapped resistor AR. The armature |5|A is connected in a circuit |63 which extends between the previously described conductor |59 and an adjustable tap provided on the resistor AR.

In order to simplify the description of the operation of this bridge circuit, we assign the reference character RI to that portion of the resistor AR which comprises one reference resistor lying on one side of the tap connected to the circuit |63 and the reference character R2 to that portion of the resistor AR which comprises another reference resistor lying on the opposite side thereof. As is well known to those skilled in the art, whenever the ratio of resistance of the reference resistors RI to R2 equals the ratio 0f resistance between the arc resistance and the resistance of the ballast resistor |58, no potential will be applied across the armature |5|A, but whenever these resistance ratios vary, a voltage will be applied across the armature |5|A and current will flow through circuit |63,

Inasmuch as the resistance of the arc is a variable quantity depending upon the length of the arc which in turn may vary in terms of the rate of consumption of the electrodes |4 and |26 and the rate at which they are fed toward each other, it will be seen that although the bridge circuit just described may be initially balanced so as to provide a zero potential on the armature |5|A, this balanced condition will soon be upset by the lengthening of the arc as the electrodes I4 and |26 are consumed. The ratio of the arc resistance to the resistance of the ballast resistor |58 will therefore be larger than the ratio of the resistance of the reference resistor Rl to the resistance of the reference resistor R2 so that a. potential is applied across the terminals of the motor armature |5|A and cause the motor |5| to revolve. We prefer to arrange the relative polarities of the field and armature of the motor |5| and the gearing connecting the motor |5| to the shaft |52 in such fashion that the motor |5| will rotate in such direction as to re-establish a balanced condition of the previously described bridge circuit.

Rotation of the motor |5| will thereforA advance the negative electrodel |26 toward the positive electrode I4 and the reduction in the length of arc thus caused will reduce the arc resistance and cause the ratio of arc resistance to the resistance of the ballast resistor |58 to approach equality with the ratio of the resistance of RI to R2. As soon as equality between these ratios is re-established, current will cease to llow through the armature ISIA and the motor I5| will stop feeding the negative electrode |26 toward the positive electrode I4. Conversely, if the bridge circuit is unbalanced due to the arc length being too short, the current will flow through the armature |5|A in a direction opposite to that just described so as to cause the reversible motor I5I to rotate in the opposite direction and cause separation of the electrodes and an increase in the length of the arc.

It is therefore apparent that the feeding mechanism for the negative electrode |26 and the controls therefor operate in such a fashion as to maintain the length of arc established between the electrodes |26 and I4 at a constant value. It is further apparent that the length of this arc and consequently its resistance is determined by the ratio of the referencesresistors RI and R2, and that by moving the point of attachment of the circuit |63 to the resistor AR so as to vary the ratio between the resistors RI and R2, the length of the arc which will be maintained by this feeding and control mechanism may be adjusted at will. Variations in the power used by the arc and by the ballast resistor |58 are caused solely by changes in the resistance of the arc when it is connected to a constant potential supply so that the feeding controls just described therefore operate to adjust and maintain the power used at a constant value.

The mechanism just described also operates to strike the arc when the lamp is rst connected to the power supply. Until such time as the arc is established between the electrodes I4 and |26, the arc resistance is substantially infinite and the feeding motor |5I is accordingly energized in such fashion as to shorten the arc as soon as the lamp is connected to the supply lines. As soon, however, as the electrodes are brought into contact with each other, the arc resistance decreases instantly to a value much lower than that for which the feeding controls are set and the feeding motor |5| is immediately reversed to draw the electrodes apart and thus strike the arc.

It will be noted that by incorporating in a projection type arc lamp the mechanisms and controls as described hereinbefore, we have provided a means for maintaining the arc thereof at a constant length and resistance so as to maintain a uniform quality and quantity of illumination by regulating the power used in the arc, and have provided means also for rotating the positive electrode so as to maintain a symmetrical crater in the end thereof, together with means for automatically feeding the positive electrode in such a fashion that the crater thereof is continuously maintained at an optimum location relative to the optical system employed in the lamp.

While we have shown and described the preferred embodiment of our invention, we do not desire to be limited to any of the details of construction shown or described herein, except as defined in the appended claims.

We claim:

1. In an arc lamp employing a pair of electrodes which are progressively consumed by an electric arc established between arcing ends thereof. the combination of: a ballast resistor; an arc circuit connecting said electrodes and said ballast resistor in series relationship to a source of power; -a pair of reference resistors; a reference circuit connecting said reference resistors in series relationship across said arc circuit; an electromotive feeding means for feeding said electrodes toward each other; and a control circuit connecting said electromotive feeding means between a point in said reference circuit common to both of said reference resistors and a point in said arc circuit common to one of said electrodes and said ballast resistor whereby said electromotive feeding means operates as a bal-` ance responsive element in a Wheatstone bridge circuit to strike said arc and to maintain the length of said arc at a substantially constant value.

2. In an arc lamp employing a rotating electrode, the combination of a frame; an electrode holder for loosely rotatably supporting said electrode on said frame; a pair of electrode brushes slidably mounted on said frame and disposed on opposite sides of said electrode for engagement therewith to conduct an electric current thereto; a non-yieldable, manually operable means for sliding one of said brushes into non-yielding contact with said electrode; and a resilient means for yieldably sliding said other brush into contact with said electrode and for forcing said loosely supported electrode against said first non-yieldable brush.

3. In an arc lamp employing ka rotating electrode, the combination of: a frame; an electrode holder for loosely rotatably supporting said electrode on said frame; a pair of electrode brushes slidably mounted on said frame and disposed on opposite sides of said electrode for engagement therewith to conduct an electric current thereto; a non-yieldable manually operable means for sliding one of said brushes into non-yielding contact with said electrode; a resilient means for yieldably sliding said other brush into contact with said electrode and for forcing said loosely supported electrode against said first non-yieldable' brush; and a guide means forming a part of said yieldably slided brush for guiding the rotation of said loosely supported electrode.

4. In an arc lamp employing a rotating electrode, the combination of 2 a frame; an electrode holder for loosely rotatably supporting said electrode on said frame; a pair of electrode brushes having an elongated electrode contacting surface for conducting an electric current to said electrode; means slidably supporting said brushes upon said frame and disposing said brushes on opposite sides of said electrode near an arcing end thereof; a non-yieldable manually operable means for sliding one of said brushes into nonyielding contact with said electrode; a resilient means for yieldably sliding said other brush into contact with said electrode and for forcing said loosely supported electrode against said first nonyieldable brush; and a guide means forming a part of said yieldably slided brush for guiding the rotation of said loosely supported electrode.

5. In an 4arc lamp employing a rotating electrode, the combination of: a frame; an electrode holder for loosely rotatably supporting said electrode on said frame; an electrode brush having an elongated plane electrode contacting surface for conducting an electric current to said electrode; another electrode brush having a pair of elongated plane electrode contacting surfaces disposed angularly relative to each other; means slidably supporting said brushes upon said frame and disposing said brushes on opposite sides o1' said electrode near an arcing end thereof; a nonyieldable manually operable means for sliding said rst named brush into non-yielding contact with said electrode; and a resilient means for yieldably sliding said other brush into contact with said electrode and for forcing said loosely supported electrode against said first non-yieldable brush, whereby the three elongated plane electrode contacting surfaces operate to guide the rotation of said loosely supported electrode.

PETER MOLE. ELMER C. RICHARDSON. FIELDING C. COATES. 

